A cured product of epoxy resin is superior in many aspects such as dimensional stability, mechanical strength, electrical insulating properties, heat resistance, water resistance and chemical resistance. However, the cured product of epoxy resin is poor in fracture toughness and may show very brittle properties, and such properties are often problematic in various uses.
As a means to solve these problems, incorporation of rubber components into the epoxy resin has been conventionally conducted. In particular, a method of incorporating rubber-like polymer particles previously prepared in the form of particles by using a polymerization method in an aqueous medium, represented by emulsion polymerization, dispersion polymerization and suspension polymerization, is considered advantageous in principle in that a dispersed state is hardly changed under blending and curing conditions, a continuous phase of a cured product of epoxy resin is not contaminated with rubber components by previously crosslinking the rubber components, so that heat resistance and toughness are hardly reduced, as compared with a method of forming a dispersed phase of rubber components in a continuous phase of a cured product of epoxy resin by causing phase separation during a curing process after dissolving and mixing non-crosslinked rubber components with epoxy resin, and thus the following various production methods have been proposed.    (1) A method which comprises milling a coagulated material of a rubber-like polymer latex and then mixing it with an epoxy resin (for example, JP-A 5-295237 and Japanese Patent No. 2751071).    (2) A method which comprises mixing a rubber-like polymer latex with an epoxy resin and then distilling water away to give a mixture (see, for example, JP-A 6-107910).    (3) A method which comprises mixing a rubber-like polymer latex with an epoxy resin in the presence of an organic solvent to give a mixture (see, for example, U.S. Pat. No. 4,778,851).
Usually when rubber-like polymer particles obtained as aqueous latex are to be mixed and dispersed in an epoxy resin, the rubber-like polymer should be separated from water.
In the method (1), the rubber-like polymer is separated from water by isolating it once as a coagulated material, but handling of this material and the step of mixing it with an epoxy resin are complicated and industrially not preferable. When the rubber-like polymer removed once as a coagulated material is to be mixed and re-dispersed in an epoxy resin, the rubber-like polymer particles are hardly re-dispersed in the form of primary particles in the epoxy resin, even by using a milling or dispersing procedure with significant mechanical shear force.
In the method (2), the epoxy resin is hardly mixed with water so that a part of the resin not mixed with water is dried to form a resin chunk, which unless removed, adversely affects qualities. In addition, a large amount of water should be removed in the presence of the epoxy resin, to make the procedure difficult.
In the method (3), while mixing the rubber-like latex with the epoxy resin, a large amount of water (an excess of water which cannot be dissolved in an organic solvent) present in the mixture in the system together with an organic solvent should be separated or distilled away, but the separation of the aqueous layer from the organic solvent layer requires much time such as whole day, or is substantially difficult because the organic solvent layer and the aqueous layer form a stable emulsified or suspended state. When water is to be distilled away, a large amount of energy is necessary, and water-soluble contaminants such as an emulsifier, auxiliary materials etc. used usually in production of the rubber-like polymer latex remain in the composition to make it inferior in qualities. Accordingly, removal of water by either method of separating or distilling water is troublesome and industrially not preferable.